This invention pertains to certain novel tocopherol ester derivatives and intermediate compounds utilized in the preparation of the derivatives. More specifically, this invention pertains to xcex1-tocopherol ester compounds that exhibit antiproliferative and growth inhibitory effects on breast cancer cell lines and to substututed dicarboxylic acids and anhydrides.
Selenium and xcex1-tocopherol (Vitamin E) are two antioxidants that have attracted great attention due to their cancer chemo-preventive activity showing strong synergism. See, for example, Ip, J. Nutr. 128, 1845-1854 (1998); Clark, et al., J. Am. Med. Assoc. 276, 1957-1963 (1996); El-Bayoumy, Mutation Research 475, 123-139 (2001); Alaejos, et al., Nutrition 16, 376-383 (2000); Burton, et al., Arch. Biochem. Biophys. 221, 281-290 (1983), and Burton, et al., Acc. Chem. Res. 19,194-201. Thus, the impact of selenium deficiency on cancer risk seemed to be more profound at low serum vitamin E concentrations as reported by Salonen, et al., Br. Med. J. 290, 417-420 (1985), Willett, et al., Lancet July 16; 2(8342):130-134 (1983), and Shamberger, et al, Arch. Environ Health 31, 231-235 (1976).
The reported synergism may be attributed to the redox recycling of the antioxidants [Chaudiere, et al., Food Chem. Toxicology 37, 949-962 (1999) and Ip, Federation Proc. 44, 2573-2578 (1985)] although d-xcex1-tocopherol is suggested to develop an environment of decreased oxidative stress where the anticarcinogenic action of selenium is potentiated through specific molecular mechanisms. Other mechanisms by which selenium compounds spare xcex1-tocopherol in the cells also have been suggested by Li, et al., FEBS Letters 508, 489-492 (2001). Previous studies on the cell growth inhibitory effects of vitamin E compounds and derivatives have shown that RRR-xcex1-tocopheryl succinate (VES) has the most potent apoptotic effect in vitro [13. Yu, et al., Nutr. Cancer 33, 26-32 (1999)]. As reported by Cheeseman, et al., Free Radic Biol Med. 19, 591-598 (1995) and Papas, Tocopherols and Tocotrienols, in A. M. Papas (Ed.) Antioxidant Status, Diet, and Health, pp. CRC Press, Boca Raton, USA, (1998), the succinate derivative of tocopherol is used extensively as a food additive or nutritional supplement because it is more stable than free tocopherol and equally bioavailable in healthy humans. Although the formation of the ester bond blocks the 6-hydroxyl group of the d-xcex1-tocopherol and inhibits the antioxidant activity, VES can be hydrolyzed retaining the active free d-xcex1-tocopherol antioxidant activity.
According to Yu, et al., Nutr. Cancer 33, 26-32, this pro-apoptotic effect of VES has not been shared by pure d-xcex1-tocopherol, an effect that can be attributed to the structural modification of the molecule of tocopherol by the succinic esteric bond. This is suggested to enable the esterified form to interact with specific molecules in the biologic process of signal generation and transduction [Yu, et al., Nutr. Cancer 27, 267-278 (1997) and Turley, et al., Cancer Res. 57, 881-890 (1997)]. It appears from the accumulated evidence [Jiang, et al., Bioch. Biophys. Res. Commun. 194, 836-841 (1993); Kaeck, et al., Biochem. Pharmacol. 53, 921-926 (1997); Lu, et al., Carcinogenesis 17, 1903-1907 (1996); Lu, et al., Biochem. Pharmacol. 49, 1531-1535 (1994); Lu, et al., Biochem. Pharmacol. 50, 213-219 (1995); and Wilson, et al., Biochem. Pharmacol. 43, 1137-1141 (1992)] that the chemical form of selenium is a very important factor in eliciting defined cellular responses in the in vitro system. Aromatic selenium compounds have been preferred to other types of organic selenium compounds due to their higher stability and lower toxicity [Ip, J. Nutr. 128, 1845-1854 (1998); Ganther, et al., Tetrahedron 53, 12299-12310 (1997); and Ganther, et al., Bioorg. Med. Chem. 9,1459-1466 (2000). Specific molecular mechanisms of their preventive action remain to be elucidated.
We have discovered that certain derivatives of xcex1-tocopherol exhibit cell growth inhibitory properties such as antiproliferative and growth inhibitory effect on breast cancer cell lines. Thus, the present invention provides xcex1-tocopherol ester compounds having formula (I): 
wherein X is a chain of 2 or 3 carbon atoms, e.g., ethylene and trimethylene, joining the 2 carbonyl groups to which they are bonded; and R1 is a group having the formula xe2x80x94Yxe2x80x94R4 wherein Y is a selenium, tellurium or sulfur atom and R4 is an alkyl cycloalkyl or aryl radical. Certain of the compounds of formula (I) have shown significant antiproliferative and growth inhibitory effect on breast cancer cell lines.
Another embodiment of the present invention is a process for the preparation of compounds of formula (I) by the steps comprising:
(1) contacting a compound having the formula Zxe2x80x94Yxe2x80x94R4with a dicarboxylic acid anhydride having the formula: 
xe2x80x83in the presence of an alkanol having the formula R3xe2x80x94OH to obtain an ester having the formula: 
(2) separating the ester of formula (III) produced in step (1) from the alkanol solvent;
(3) contacting the ester of formula (III) with aqueous inorganic acid to convert the ester to the corresponding dicarboxylic acid;
(4) contacting the dicarboxylic acid formed in step (3) with acetic anhydride to convert the dicarboxylic acid to an anhydride having the formula: 
xe2x80x83and
(5) contacting the anhydride of formula (IV) with RRR-xcex1-tocopherol to obtain an xcex1-tocopherol ester having formula (I);
wherein X and R1 are defined above; R3 is alkyl of 1 to 3 carbon atoms and Z is an alkali metal.
The compounds having the formula (V) are especially preferred: 
wherein R1 and R2 each is hydrogen or phenylselenyl, i.e., phenyl-Se-, provided that R1xe2x89xa0R2, i.e., one of R1 and R2 is phenylselanyl and the other is hydrogen. The compounds of formula (V) exhibit antiproliferative and growth inhibitory effects on breast cancer cell lines superior to the effect of xcex1-tocopheryl succinate and its equimolar combination with 2-phenylselanyl-succinic acid.
The compounds of our invention may be prepared by means of known procedures using available materials. For example, the compounds may be prepared by contacting RRR-xcex1-tocopherol with an anhydride having formula (IV) in the presence of an acidic catalyst and a hydrocarbon solvent. For example, phenylselenosuccinic anhydride may be contacted with xcex1-tocopherol in the presence of zinc chloride and toluene. The compounds of formula (I) typically comprise a mixture of compounds, e.g., phenylselanyl-succinic acid RRR-xcex1-tocopherol having the structure 
typically comprises a mixture of the 2-phenylselanyl compound [Compound (I)xe2x80x94R1=H, R2=phenylselanyl] and the 4-phenylselanyl compound [Compound (I)xe2x80x94R1=phenylselanyl, R2=H]. The anhydride having formula (IV) may be obtained by contacting the corresponding dicarboxylic acid with acetic anhydride at elevated temperatures. The dicarboxylic acid is obtained by the hydrolysis of an ester of formula (III) at elevated temperature using a mineral acid such as hydrochloric acid. Ester (III) X can be prepared by first contacting a compound having the formula R4xe2x80x94Yxe2x80x94R4, e.g., diphenyldiselenide, with an alkali metal borohydride, e.g., sodium borohydride in the presence of an alkanol solvent, e.g., methanol, and then adding bromosuccinic anhydride to the alkanol soluton of the Zxe2x80x94Yxe2x80x94R4 intermediate. Anhydride (II) can be obtained by heating bromosuccinic acid with acetic anhydride.
The groups represented by R4 may be unsubstituted or substituted alkyl, cycloalkyl or aryl containing up to about 20 carbon atoms. The alkyl radicals preferably are alkyl of 1 to about 6 carbon atoms. Examples of the aryl groups include phenyl and phenyl substituted with one or two substituents selected from alkyl, alkoxy, nitro, halogen and the like. R1 most preferably represents a phenylselenyl group.